The present invention relates to a four-wheel steering apparatus for use in motor vehicles and the like.
Four-wheel steering apparatus for use in motor vehicles comprise a front-wheel steering device and a rear-wheel steering device. Rear-wheel steering devices are known which comprise a rear-wheel steering bar mechanically separated from the front-wheel steering device and movable by an electric motor in accordance with the speed of the vehicle and the steering amount of the front wheels for steering the rear wheels (see U.S. Pat. No. 4,669,744 and Unexamined Japanese Patent Publication HEI 1-136876).
With such rear-wheel steering devices, the relative steering direction of the rear wheels (the steering direction of the rear wheels relative to that of the front wheels) and the maximum permissible steering amount are controlled according to the speed of the vehicle.
FIG. 9 shows an example of relationship of the vehicle speed to the relative steering direction and steering amount (steering angle) of the rear wheels. In the drawing, the vehicle speed (km/h) is plotted as abscissa vs. the steering amount (deg.) as ordinate. Represented by the steering amount of 0 is the neutral state, by the positive steering amount a steering direction of the rear wheels which is the same as that of the front wheels, i.e., steering of the same phase, and by the negative steering amount a steering direction of the rear wheels which is reverse to that of the front wheels, i.e., steering of reverse phase. The solidline curve A represents maximum permissible steering amounts at varying vehicle speeds. When the vehicle speed is low in this case, reverse-phase steering is done, and the maximum permissible steering amount increases as the vehicle speed decreases. When the vehicle speed is high, same-phase steering is done, and the maximum permissible steering amount increases to a value and then decreases with increasing vehicle speed. Within the range of maximum permissible steering amounts at varying vehicle speeds, i.e., within the range surrounded by the curve A and the abscissa, the steering amount of the rear wheels is controlled in accordance with the steering amount of the front wheels. In either case of steering of reverse phase or same phase, the actual steering direction of the rear wheels changes with the steering direction of the front wheels. Accordingly, it follows that the curve A of FIG. 9 represents maximum permissible steering amounts of the rear wheels at varying vehicle speeds in either of right and left directions.
With the rear-wheel steering device described, the amount of movement of the rear-wheel steering bar is mechanically (structurally) limited, and the foregoing mode of control is effected within the mechanical limits. With reference to FIG. 9, the two dot-and-dash lines B and C parallel to the abscissa represent steering amounts when the rear-wheel steering bar is moved to the right and left mechanical limits. Naturally, these amounts are slightly greater than the greatest value of maximum permissible steering amounts of the curve A. Mechanically, the rear-wheel steering bar is movable by the mechanically limited maximum amount irrespective of the vehicle speed, so that the rear wheels can be steered between the line B and the line C.
In the case where the rear-wheel steering device operates normally in its entirety, the steering amount of the rear wheels will not exceed the range surrounded by the curve A and abscissa of FIG. 9. In the event of a runaway of the electric motor, however, the steering amount of the rear wheels is likely to greatly exceed the maximum permissible amount at a particular vehicle speed since mechanically the rear wheels can be steered between the two lines B and C, hence a great hazard especially during high-speed running.
The rear-wheel steering devices include those wherein the rear-wheel steering bar is movable hydraulically as by a hydraulic motor or cylinder as disclosed, for example, in Unexamined Japanese Patent Publication HEI 2-63974. The same problem as described above is encountered also with these devices.